1. Field of the Invention
The present invention relates to a semiconductor laser device, in particular to a dual-wavelength semiconductor laser device having two semiconductor lasers formed on a common semiconductor substrate and to a method for fabricating the same.
2. Description of the Prior Art
Today, a semiconductor laser device is widely used in various fields. Especially in recent years, a dual-wavelength semiconductor laser device having two semiconductor laser elements which have different oscillation wavelengths and are formed on a common semiconductor substrate has been in the limelight and has enjoyed a rapidly increasing demand as a light source of an optical pickup device for recording/playback of DVD and CD.
An example of a conventional dual-wavelength semiconductor laser device having two semiconductor laser elements is shown in FIG. 7 (see Japanese Published Unexamined Patent Application No. 2004-349286). FIG. 7 shows a configuration of a monolithic dual-wavelength semiconductor laser device having an infrared laser element A for emitting light in a band of 780 nm and a red laser element B for emitting light in a band of 650 nm on a common substrate. The infrared laser element A and the red laser element B are separated from each other by a separation groove C which reaches the substrate.
First, a structure of the infrared laser element A will be described.
The infrared laser element A includes an N-type buffer layer 1202 made of GaAs (the amount of doped Si:1.0×1018 cm−3), an N-type clad layer 1203 made of (Al0.3Ga0.7)0.5In0.5P (the amount of doped Si:1.0×1018cm−3), an undoped infrared active layer 1204, a P-type first clad layer 1205 made of (Al0.3Ga0.7)0.5In0.5P (the amount of doped Zn:3×1017 cm−3), a P-type etching stopper 1206 made of Ga0.5In0.5P (the amount of doped Zn:1×1018 cm−3), a P-type second clad layer 1207 made of (Al0.3Ga0.7)0.5In0.5P (the amount of doped Zn:1×1018 cm−3), and a P-type cap layer 1208 made of Ga0.5In0.5P which are stacked in sequence from the underside on a substrate 1201 made of N-type GaAs. It is to be noted that, the N-type buffer layer 1202 is provided to improve the crystalline quality of the N-type clad layer 1203.
The P-type second clad layer 1207 is processed to have a striped ridge shape. On the P-type second clad layer 1207, the striped P-type cap layer 1208 is provided. Moreover, on both sides of the P-type second clad layer 1207, an N-type current block layer 1209 made of Al0.5In0.5P (the amount of doped Si: 1.0×1018 cm−3) is provided. On the P-type cap layer 1208 and the N-type current block layer 1209, a contact layer 1210 made of P-type GaAs (the amount of doped Zn: 7×1018 cm−3) is provided. On an upper surface of the contact layer 1210, a P-side electrode 1211 is provided. On a lower surface of the substrate 1201, an N-side electrode 1215 is provided. To etch the P-type second clad layer 1207 for forming a ridge, the P-type etching stopper 1206 which is different from the P-type second clad layer 1207 in composition is provided. Using an etching process in which the etching rate of the etching stopper 1206 is considerably low allows the ridge to be formed with high dimensional accuracy.
FIG. 8 is a cross-sectional view showing a configuration of the infrared active layer 1204 of FIG. 7. As shown in the figure, the infrared active layer 1204 has a multiple quantum well structure composed of, in sequence from the underside (n side), a first optical guide layer 1104a made of undoped Al0.4Ga0.6As, a layered structure including a total of four well layers 1104b made of undoped GaAs and a total of three barrier layers 1104c made of undoped Al0.4Ga0.6As which are alternately stacked, and a second optical guide layer 1104d made of undoped Al0.4Ga0.6As.
Next, a structure of the red laser element B will be described.
As shown in FIG. 7, the red laser element B includes an N-type buffer layer 1222 made of GaAs (the amount of doped Si:1.0×1018 cm−3), an N-type clad layer 1223 made of (Al0.3Ga0.7)0.5In0.5P (the amount of doped Si: 1.0×1018 cm−3), an undoped red active layer 1224, a P-type first clad layer 1225 made of (Al0.3Ga0.7)0.5In0.5P (the amount of doped Zn:5×1017 cm−3, an etching stopper 1226 made of Ga0.5In0.5P (the amount of doped Zn:1.0×1018 cm−3), a P-type second clad layer 1227 made of (Al0.3Ga0.7)0.5In0.5P (the amount of doped Zn:1×1018 cm−3), and a P-type cap layer 1228 made of Ga0.5In0.5P which are stacked in sequence from the underside on the substrate 1201. It is to be noted that, the N-type buffer layer 1222 is provided to improve the crystalline quality of the N-type clad layer 1223.
The P-type second clad layer 1227 is processed to have a striped ridge shape. On the P-type second clad layer 1227, the striped P-type cap layer 1228 is provided. Moreover, on both sides of the P-type second clad layer 1227, an N-type current block layer 1229 made of Al0.5In0.5P (the amount of doped Si: 1.0×1018 cm−3) is provided. On the P-type cap layer 1228 and the N-type current block layer 1229, a P-type contact layer 1230 made of GaAs (the amount of doped Zn: 7×1018 cm−3) is provided. On an upper surface of the P-type contact layer 1230, a P-side electrode 1231 is provided. Also in the red laser element B, the etching stopper 1226 is different from the P-type second clad layer 1227 in composition, and a process is used in which etching is substantially stopped by the etching stopper 1226. This allows the ridge formation with good dimensional accuracy.
FIG. 9 is a cross-sectional view showing a configuration of the red active layer 1224 of FIG. 7. As shown in the figure, the red active layer 1224 has a multiple quantum well structure composed of a first optical guide layer 1224a made of undoped (Al0.5Ga0.5)0.5In0.5P, a layered structure formed by alternately stacking well layers 1224b made of undoped Ga0.5In0.5P and barrier layers 1224c made of undoped (Al0.5Ga0.5)0.5In0.5P, and a second optical guide layer 1224d made of undoped (Al0.5Ga0.5)0.5In0.5P which are stacked in sequence from the underside (n side).
A very important point in the fabrication of the conventional dual-wavelength semiconductor laser device is that the infrared laser structure is formed before the formation of the red laser structure. The reason is as follows. That is, in the red laser structure, the red active layer is made of a material containing P, for example, GaInP or AlGaInP. However, in these material films, the diffusion rate of Zn under a high temperature over 500° C. is greater than or equal to ten times as fast as in a material containing As, for example, GaAs or AlGaAs. Therefore, if the red laser structure is first formed, Zn diffuses from the P-type first clad layer 1225 and the P-type second clad layer 1227 of the red laser element into the red active layer 1224 in the subsequent crystal growth of the infrared laser structure, which may cause a change in band gap, and thus in oscillation wavelength.